The use of the electricity network as a means of transmission is known in the background art, but due to its poor performance, its use as a data transmission network has been limited to point-to-point communication at very low speeds.
This is due, among other reasons, to the fact that, in the electricity network, the connection and disconnection of apparatus generate voltage peaks and impedance variations on the line and cause serious loss of signal that varies in function of frequency and time.
Furthermore, various obstacles impede the establishment of communication between a head-end and a plurality of users, in particular due to the many changes in impedance in different frequencies and the emergence of reflections that cause the received signal to be a combination of the transmitted signal and a series of echoes that circulate through the electricity network with different attenuations and delays for each one of the users on the received signal.
Furthermore, attenuation, noise, and channel response vary dynamically in frequency and time.
All these obstacles have to date limited the use of the electricity network for full-duplex, high-speed point to multipoint communication.
On the other hand, other means of communication for the transmission of data are known in the background art, such as the use of twisted pair in telephones to establish point to point or point to multipoint communication.
In this context we cite U.S. Pat. No. 5,673,290 wherein a method of point to point transmission is described that consists of communication via a downstream channel determined by a link from the head-end to a plurality of different users, and communication via the upstream channel determined by a link from the users to the head-end, whereby the communication is made possible using a discrete digital multi-tone (DMT) transmission system and providing the coding of the digital data and the modulation of the encoded data over the discrete multi-tone signal.
Furthermore, the communication line is supervised to determine at least one line quality parameter, including noise levels in each one, and includes a multitude of sub-channels each one corresponding to an associated sub-carrier tone. The modulation system is designed to take various factors into account including detected line quality parameters, the parameters of sub-channel gains, and a masking parameter of permissible power when modulating the discrete multi-tone signal. The modulation system is also capable of dynamically activating the sub-carriers used and the quantity of data transmitted in each sub-carrier during transmission to adapt in real time to changes in individual sub-carriers.
In applications susceptible to interference, the associated bandwidths can be simply masked or silenced to prevent interference in either direction, and therefore, the signals are transmitted by sub-carriers with frequencies above or below the most significant noise levels.
Furthermore, in this document the transmission occurs in base band and the conjugated real hermitian transformation of the transmissible information is used (real Fast Fourier Transform). Due to the characteristics already described, this transmission method cannot be applied to transmission over the electricity network.
Furthermore, the method described in the patent cited above refers to point to point communication, therefore, neither its use over the electricity network nor the possibility for full duplex point to multipoint communication can be inferred.
On the other hand, point to multipoint communication systems exist such as that described in the PCT Patent Number WO96/37062 where the transmission line can be coaxial cable, fibre optic or similar, which uses orthogonal frequency division multiple access modulation system (OFDM), a modulation system that is well known in the background art, and to which a cyclic prefix is added to each OFDM symbol to alleviate the effects of the multipath propagation as is well known in the state of the art. The use of the cyclic prefix with the OFDM modulation can be encompassed by the DMT modulation used in the previous document and equally widely used in the state of the art.
The document describes how channels are established over respective sub-carrier groups, so that each user is assigned a specific group of tones so that the hardware and the complexity involved in realizing the discrete Fourier transformation is substantially reduced, however, as a fixed system it does not allow the assignation of different sub-carriers to the users depending on the prevailing frequency and time conditions in each channel, even when, as described in the case of U.S. Pat. No. 5,673,290, the individual sub-carriers can connect or disconnect to avoid interferences.
Furthermore, it uses a remote loop to correct the frequency of local oscillators of the various user modems.
As a relevant background art we can cite Document EP-1011235-A2, which discloses an embodiment to receive multicarrier signals over power lines. It shows an Orthogonal Frequency Division Multiplexed (OFDM) power line communications system comprising a power line for distributing electricity to a plurality of premises and a communication station coupled to the power line at one of the premises, which station uses a part of the power line external to the premises as a communication medium. The communications station includes a receiver which comprises a clipping system adapted to a clip an incoming OFDM data waveform, which includes a regular impulsive noise component, so as to reduce the level of noise on the waveform.
We can also cite U.S. Pat. No. 5,815,488 and U.S. Pat. No. 5,828,660 regarding point to multipoint communication.
Neither in these documents is there a description of the adaptation for the transmission using the electricity network.
Furthermore, none of the documents cited previously concerns the transmission for multiple users, nor how to maximize the throughput of the upstream and downstream channels in the electricity network.